I believe there are some thousend freeware tools that monitor some sort of performance. Unfortunately most of those monitors are not aware of AIX and not aware of pSeries hardware. Hence they will fail to monitor more than the basics. Especially when it comes to RSCT and Kernel those tools often are not up to the job. Things might get worse when you have to compile the monitor yourself. That all is unfortunate but at the same time unneccessary. As an AIX admin you'd probably use one of the monitors from the IBM wiki: IBM developerWorks: Wikis - AIX - Performance Other Tools

A very fast and easy montoring can be set up with nmon. It can collect perf data into a file that can be analysed with the nmon analyser. So to set up monitoring on a standalone server in two steps you'd
1. Install nmon on target.
2. Call nmon from crontab :

Code:

1 0 * * * nmon -F $(date +\%d\%m\%Y).nmon -I 0.001 -s 300 -c 288 -m /<datatarget_directory>

You are done. This will collect a 24 hour period of performance data with a five minute intervall into a file every day.
Two more hints for using nmon
- Nmon uses public API hence no need to run it as root.
- If nmon stops writing into the file there might be an error with the device that is being queried. In that case either remove the error or prevent nmon from querying that device (by commandline option).

@OP: You are right: to find out if/how the new app affects performance of your system you need to know what the load was before the new app was installed. So for comparison you need to uninstall the app, monitor, then install the app and monitor again. However if monitoring with the app reveals that performance is still ok you might simply leave it.